This invention relates to a process for the preparation of coatings of polymers having hydantoin ring structures by the reaction of blocked isocyanates containing carbodiimide and/or optionally uretone imine groups with .alpha.,.beta.-unsaturated dicarboxylic acids or with derivatives thereof said reaction being carried out on the substrates to be coated.
It is known that substituted hydantoins are obtained when .alpha.,.beta.-unsaturated carboxylic acid derivatives are reacted with isocyanates (DE-OS No. 2 654 112) and that improvements in the resulting products may be obtained if the isocyanates used are converted into carbodiimides before the preparation of the hydantoins (e.g. DE-OS Nos. 3 144 700 and 3 247 350 which equals U.S. Pat. No. 4,517,353). The polyhydantoins may be used, for example, as heat resistant plastics, especially in the field of electrical insulation (e.g. FR-PS No. 1 484 694).
It is known from DE-OS No. 3 247 350 that isocyanates which are partially blocked with lactams can be converted into carbodiimides which in turn can be reacted with .alpha.,.beta.-unsaturated carboxylic acids to form polyhydantoins. In this process, the .alpha.,.beta.-unsaturated compound is slowly added with cooling.
It was therefore not to be expected that mixtures which are stable in storage at normal temperature could be prepared from the components according to the invention.
A problem generally encountered in the production of films and coatings based on polycarbodiimides is that it is necessary to start with relatively large molecular units so that the stoving process for producing the polymer which forms the coating or film can be carried out with relatively few bonding or cross-linking reactions and can be completed within a short time. This means that the lacquer solution has a high viscosity and a relatively low solids content. If the compounds are composed of low molecular weight units, they must undergo a larger number of cross-linking or bonding reactions which require a longer reaction time which cannot be simply compensated by increasing the temperature as this might lead to the formation of blisters or surface defects. Another possibility of obtaining the lacquer from preliminary stages with smaller molecules so that a low viscosity lacquer containing a larger amount of film forming material could be obtained would be to increase the reactivity considerably by means of special catalysts, but this could have a disadvantageous effect on the viscosity during a continuous process.
It has now been found that a film of polymer lacquer can be obtained from low molecular weight starting materials which can be applied directly to the substrate without the elaborate process of preparing an intermediate polymer stage.